A. Field of Invention
This invention pertains to the art of methods and apparatuses of producing power transmission belts and more particularly to a method of preventing or minimizing axial movement of underlying elastomer and fabric materials as tensile cord is added to the belt.
B. Description of the Related Art
Elastomeric belt products such as synchronous power transmission belts often include a tensile member layer comprising one or more cords. The cord is wound onto underlying cylindrical layers of belt material prior to the belt being molded and cured.
U.S. Pat. No. 6,390,406 discloses a method and apparatus for applying accurate lengths of a cord to a rotating structure during the construction of an elastomeric belt. The cord is wound onto underlying material in a helical pattern. The cord-laying wheel moves transversely from side to side as the underlying material is rotated either on a mandrel or on a pair of cylinders. During the cord-laying process on a pair of cylinders, the underlying material tends to move transversely due to tension from the cord. The underlying material often has insufficient strength and stiffness to support the cord layer, resulting in stretch of the underlying material along its circumference.
Most synchronous power transmission belts are made on a cylindrical mold drum. Layers of elastomer and fabric are placed on the drum and one or more tensile cords are helically wrapped around the drum. The cords are accurately placed on the drum by a cord laying wheel which is moved along the axis of the drum as the drum rotates, winding the cords in an accurate helix. The drum may also be used as a mold when the belt teeth are formed and the belt is vulcanized. The circumference of the drum must closely match the desired circumference of the belt, so that a different drum is required for each desired belt length.
In an alternative process, belts are cured incrementally in a press or roto-cure machine without a drum that matches the belt circumference. In the alternative process, the cord is wound around two smaller drums that are held at a fixed distance such that the circumference of the cord path around the drums is equal to the circumference that it would have had if wound on a single drum. As the drums rotate, the cord travels from one drum to the next and back. The cords are placed on one drum or in the span between drums by a cord wheel that moves along the axis of the drums as they rotate, again winding the cords in a helix. Because they are not fixed to the drums, the cords, fabric and elastomer layers of the belt can move axially along the drum. The traditional methods of controlling the axial movement (tracking) include using crowned or tapered drums, crossing the axes of the drums, the addition of angled rollers between the drums which drive by friction on the face of the belt, and tentering machines which grip the edge of the belt. Each of these methods causes a variation in the length of the cords when they are wound onto the drums, so they are not suitable for precision-molded synchronous power transmission belts. The circumference of the underlying layers must be close to the belt path circumference about the drums. If the underlying layers are too long, or become stretched along the circumference, they can become wrinkled or folded as the drums rotate. In the alternative process, belts are cured incrementally in a press or roto-cure machine where the belt is placed around two or more small drums that are held at a fixed distance such that the circumference of the cord path around the drums is approximately equal to the circumference that it would have had if placed on a single drum. The drums are rotated to position incremental sections of the belt for molding and vulcanization. Here too, the belt can move axially along the drum.
The present invention provides an improved apparatus and improved methods for minimizing axial movement of the belt as the cord is being wound, as additional layers are added, and as the belt is advanced during molding and vulcanization. The present invention is simple in design and effective in use to provide tracking control without distorting the path of the cord wound onto the belt. It also prevents stretching of the underlying belt material as the drums rotate.